The Scottish Parliament seems set for a showdown with the Scotland Office and Westminster after cross-party MSPs yesterday called for the return of 6000 square miles of Scottish fisheries handed over earlier this year to England.

Holyrood's Tory-chaired rural affairs committee - where Labour is in the minority - produced a critical report on the fisheries boundary issue and recommended that Scottish Secretary Dr John Reid should have the controversial decision reversed by Westminster, as demanded by Scotland's fishing fleet. Only the Government in London has the power to change the boundary again.

Labour's coalition partners in Scotland, the Liberal Democrats, abandoned the joint party line and voted with the SNP and Tories. By doing so they were adding force to moves in the Commons by Lib Dem MP Archy Kirkwood to have Westminster reinstate the old boundary.

Scottish Executive spokesmen have argued since the change that technically there never was a boundary and that the decision to create one was purely for administrative reasons after devolution.

The new fisheries boundary no longer bears resemblance to the boundary which delineates Scotland's territory in oil and gas fields as formalised in 1987.

If Mr Kirkwood's move succeeds, the fisheries boundary will restore all but a few miles of the ''lost'' waters to the same boundary as that for offshore activities.

The rural affairs committee reached its decisions with a series of votes, most of them involving its four Labour MSPs being on the losing side. The significance of the Lib Dem action means that the committee now reflects majority opinion in the Scottish Parliament as a whole - and that adds considerable weight to moves to have Westminster restore the status quo.

SNP Opposition rural affairs spokesman Alasdair Morgan said: ''The pressure is mounting. The committee reflects the make-up of the chamber, which can be expected to take precisely the same line.''

The SNP's Richard Lochhead told reporters: ''The ball is now in John Reid's court. This is his biggest test since taking office. He must now demonstrate whether he is the UK Cabinet's man in Scotland or Scotland's man in the UK Cabinet.''

Nationalists suspect the Westminster move to hand jurisdiction to England was made as a pre-emptive strike against the possibility of Scotland becoming independent. They believe Westminster might try to use the change as a precedent for moving the oilfields boundary.

For the Tories, committee chairman Alex Johnstone described Westminster's decision to transfer jurisdiction over the disputed waters as a ''mistake'' and said it was disappointing that the Government had refused to heed protests and the feelings of the Scottish fishermen.

He added: ''The deputy Fisheries Minister, John Home Robertson, suggested we were over-reacting and that this an issue of no significance at all. We say that, if it is really of no significance at all, then he should accept our suggestions in the report. Westminster should take action as quickly as possible to move the boundary back.''

Dr Reid made it clear last night that the report changed nothing.

''At first glance there is nothing new in the report to warrant reconsideration of the Boundaries Order,'' his spokeswoman said.

''He has met Scottish fishermen's representatives and listened carefully to their concerns but hasn't been persuaded that the boundary poses any practical disadvantages to them. Scottish fishermen's access to the fishing grounds remains completely unchanged.

''There has been quite a bit of scaremongering about this and after all the reality is that the responsibility for regulating 140,000 square miles of British fishery limits has been transferred to the Scottish Parliament.

''This boundary has been chosen based on the median line and the principle of equidistance which is the most common means of determining boundaries along international practice.''

This rejection will infuriate Scottish fishermen who believe the initial decision was so absurd it must have been a mistake.

Scottish Fishermen's Federation chief executive Hamish Morrison welcomed the move by the Rural Affairs Committee and praised the ''workmanlike assessment of the history of this unfortunate situation''.

''It is important to keep this matter in proportion,'' he said. ''Nevertheless it is necessary to resolve this existing anomaly to prevent its continued intrusion into the conduct of more pressing matters that confront the fishing industry.

''The issue may seem complex but it is essentially straightforward. As a consequence of the devolution settlement there are now two sea boundaries between Scotland and England. The lines are separate, distinct, and in the North Sea they diverge by up to 50 miles.

''The line created by Statutory Order in April of this year defines the Scottish Parliament's area of legislative competence on fisheries matters.

''This new Order does not supersede the Statutory Order made in 1987 that defines the offshore jurisdiction of the Scottish courts. Hence, there is one boundary too many.

''This situation is so manifestly absurd that it could not have been contrived knowingly. An error or an oversight must have created this anomaly. Scottish fishermen may expect the highest standards of government but they do not expect infallibility.

''The Federation expects the UK Government to agree to a single sea boundary between Scotland and England and that the boundary line defined in 1987 should take precedence.''

Mr Morrison said it would be ludicrous if there were two land boundaries just as it was ridiculous to have two offshore boundaries.

''There are many serious issues affecting Scottish and English fishermen on which close co-operation is required with the Government. It is therefore important to resolve this boundary debacle so that fishermen and Government can focus on the all-important tasks of managing fish stocks and developing the industry.''